The reason for the offset seam in the backing is to not be right on a natural fold line, like exactly half the quilt. Just a potential stress/wear point.

Even still I have pieced backs with border prints where I want it on the natural fold. Typically for machine self-quilting and not long arm.

Yes it is fine to have only one horizontal seam. I often do. Sometime however, if it is a large quilt, I will insert a row of contrasting fabric that coordinates with the front and put it off center to make it look like the addition was intended as a design element.

I agree with Iceblossom!! An off center seam moves it from the location that most folks want to fold on, Also for me using a wide and then narrower piece also "saves" fabric - leaving you a larger single piece leftover to use elsewhere,

edited to add a pic of one where I used extra blocks on the back and it is off center

Thank you both! That is very helpful